This invention relates to thermoelectric modules and especially to low cost thermoelectric modules.
Thermoelectric devices for cooling and heating and the generation of electricity have been known for many years. A well-known technique for generating electric power is to arrange thermoelectric elements in an egg crate type module. A very successful method of making egg crate type thermoelectric modules is described in detail in U.S. Pat. No. 5,856,210 that is incorporated by reference herein. For the convenience of the reader, some of the figures from that patent have been included in this application as FIGS. 1A and B and FIGS. 3A through 7. Thermoelectric modules of the type shown in FIGS. 1A, B and C are used to convert waste heat into electricity. Since the heat energy is waste energy, one might think the resulting electrical energy is free. Such is not the case. The reason is that the cost of producing the thermoelectric module has in the past been greater than the value of the electric energy it would produce throughout its expected life. For this reason, thermoelectric modules have been utilized for the most part in situations where conventional electric power is not readily available.
It is generally believed that if the cost of thermoelectric modules can be reduced enough so that the module cost is less than the cost of the amount of conventional electricity corresponding to the electricity they would generate in a reasonable period of time, the market for these modules would increase enormously.
In this module design shown in FIGS. 1A, B and C, there are spaces for 100 elements. In some cases the two elements at the location of the electrical leads 61 and 62 of the module are shorted to make a better lead connection leaving 98 active elements. Electric current is produced by the temperature difference between the hot and cold sides of the module. The elements of the thermoelectric module are called xe2x80x9cp legsxe2x80x9d and xe2x80x9cn legsxe2x80x9d which are arranged in a checkerboard pattern and are usually connected electrically in series. Current flows from the hot side to the cold side through the n legs and from the cold side to the hot side in the p legs. Each leg of the ""210 module typically produces a potential of only about 16 millivolts (under design load) with a temperature difference of about 200 degrees C. Since the 100 legs are in series the module operates at about 1.6 Volts and produces 13 Watts of electrical power. In typical applications the thermoelectric module is sandwiched between a hot metallic surface and a cold metallic surface. An electric insulator is usually used between the hot and cold surfaces of the thermoelectric module and the hot and cold metallic surfaces to prevent electric shorting. This insulator can be either a ceramic such as Al2O3, BN or a plastic such as Kapton. These insulators are often used in conjunction with a thermal grease such as silicone oil filled with a ceramic powder such a BN or ZnO. Several companies also offer preformed compliant insulators that can be used in place of the ceramic insulators. Also, a pressure is usually applied to the module insulator system that can range from 5 lbs/in2 to 200 lbs/in2 depending on the insulator system used. This pressure is applied to minimize the thermal contact resistance between the heat transfer surfaces and the thermoelectric module. This thermal resistance across each of the electrical insulator gaps results in temperature drops on both the hot and cold sides of the module that decreases the overall system efficiency. This temperature drop can vary from near zero to 30xc2x0 C. or 40xc2x0 C. or more on each side of the module, depending on the material used, the applied pressure and the heat flux flowing through the module. The single greatest cost of most egg crate type thermoelectric modules is the cost of the thermoelectric material making up the n and p legs. These elements represent about 40 to 60 percent of the cost of the module.
A good thermoelectric material is measured by its xe2x80x9cfigure of merrit or Z defined as:
Z=S2/xcfx81K
where S is the Seebeck coefficient, xcfx81 is the electrical resistivity, and K is the thermal conductivity. The Seebeck coefficient is further defined as the ratio of the open-circuit voltage to the temperature difference between the hot and cold junctions of a circuit exhibiting the Seebeck effect, or
S=V/(Thxe2x88x92Tc).
Therefore, a good thermoelectric material is a material with large values of S and low values of xcfx81 and K.
Thermoelectric materials currently in use today include the materials listed below with their figures of merit shown:
Prior art thermoelectric elements making up n and p legs are typically in the shape of rectangular prisms as indicated in FIGS. 1A, B and C. The walls of the egg crate typically have been made as thin as feasible taking into consideration structural and electrical insulation requirements.
What is needed is a less costly thermoelectric module.
The present invention provides a low-cost thermoelectric module utilizing a greatly reduced quantity of thermoelectric material as compared to similar prior art thermoelectric modules. This reduction is accomplished by taking advantage of a known but not utilized relationship that permits a substantial reduction in the quantity of thermoelectric material in a module without any significant reduction in the efficiency, voltage or power output of the module. An egg crate design is utilized in the present invention. However, contrary to prior art the walls of the egg crate in the parts of the module separating the thermoelectric elements are much thicker so that the total cross sectional area of the elements is less than 75 percent of the total module cross sectional area. In a preferred embodiment the cross sectional area of the elements is only about 50 percent of the total module cross sectional area. Above and below the thermoelectric elements the walls of the egg crate are tapered becoming very thin at the top and bottom of the eggcrate. The spaces above and below the elements are filled with a high heat and electric conducting material such as aluminum. This produces funnel-shaped conductors funneling heat and electric current into and out of each of the thermoelectric elements. In a preferred embodiment a module that is the same size as a popular prior art module uses only 44 percent as much expensive thermoelectric material with no significant reduction in performance. The module has the same number of elements but each element is much smaller than the elements in the prior art module. As a consequence of the funnel-shaped conductors, approximately the same amount of heat flows through the smaller thermoelectric elements as compared to the prior art standard size elements. The payoff to this approach is that the heat flux through the hot and cold whole module surfaces can be maintained while producing the same power output with about half the thermoelectric material or less. Avoiding any significant increase in heat flux through the whole module surface means avoiding increases in temperature drops normally encountered at the hot and cold module surfaces. This invention produces a module cost reduction of about 30 percent as measured in cost per Watt of thermoelectric generating capacity.